1. Field of the Invention (Technical Field)
The present invention relates to the detection of fires in closed compartments such as aircraft cargo bays and buildings using fire alarm algorithms and sensors for monitoring fire signatures.
2. Description of Related Art
Fire signatures for flaming and smoldering fires have included temperature, smoke, and chemical species. The chemical species may include oxygen (O2, hereafter O2), carbon monoxide (CO), carbon dioxide (CO2, hereafter CO2), water vapor (H2O, hereafter H2O), nitric oxide (NO), hydrogen cyanide (HCN), acetylene (C2H2, hereafter C2H2), etc. Fire alarm algorithms based on fire signatures are developed using intuition (e.g., threshold and rate of increase), systematic methods (i.e,. involving mathematical formula), and variable methods (e.g., artificial neural network). Simple algorithms are based on thresholds for maximum values, rates of increase, and combinations thereof.
Fire detection systems of current aircraft cargo compartments are primarily smoke detectors. The false alarm rates, defined as the percentage of alarms with no verified smoke in the cargo compartment, are as high as 99 percent. The cost of a false alarm is estimated between $30,000 to $50,000 per incident (D. Blake., “Aircraft cargo compartment smoke detector alarm incidents on U.S.-registered aircraft, 1974–1999, DOT/FAA/AR-TN00/29, 2000). Moreover, regulations mandate that the alarm sounds within one minute after the onset of a fire condition. Pilots may have only about ten to fifteen minutes in which to land before smoke or damage to the structure from an uncontained fire prevents the pilot from controlling the aircraft. Reducing the time to alarm will allow pilot to suppress the fire at an earlier stage and permit more time to land the aircraft safely.
Several fire detection systems have been developed to reduce false alarms and decrease the time response of smoke detectors. Several approaches have been taken to improve the performance of smoke detectors. Other detection systems have taken a totally different approach to fire detection.
J. A. Milke, “Monitoring multiple aspects of fire signatures for discriminating fire detection,” Fire Technology 35, 195–209 (August 1999), uses a pair of CO and CO2 detectors to identify flaming and non-flaming fires. Flaming fires is detected by the CO2 threshold concentration or rate of increase. Non-flaming fire is detected by the rate of increase of CO or CO2. Reduction in false alarms and detection times were observed when compared to a commercial smoke detector.
D. T. Gottuk, et al., “Advanced fire detection using multi-signature alarm algorithms,” Fire Safety Journal 37, 381–394 (2002), use a fire alarm algorithm based on the product of CO absolute concentration and smoke obscuration level. Results have shown improvements over both ionization and photoelectric smoke detectors alone in terms of reduction in nuisance alarms and response times.
B. C. Hagen, et al., “The use of gaseous fire signatures as a mean to detect fires,” Fire Safety Journal 34, 55–67 (2000), use a fire detection system comprising two Taguchi sensors (820 and 822), two gas sensors (CO and CO2), and temperature. The fire alarm algorithm uses threshold values to classify flaming fire (when CO2>210 ppm and temperature>40° C.), smoldering fire (when CO>17 ppm, and CO2>22 ppm, and Taguchi 822>0.270V), and nuisance sources (when Taguchi 822>0.9V and Taguchi 880>0.15V). This combined system was found to perform better than two smoke detectors without introducing additional false alarms.
S. L. Rose-Pehrsson, et al., “Real-time probabilistic neural network performance and optimization for fire detection and nuisance alarm rejection,” 12th International Conference on Automatic Fire Detection (March 2001), use ionization and photoelectric detectors, CO and CO2 sensors using magnitude and slope information, and background subtraction to evaluate a fire alarm algorithm based on probabilistic neural network. Flaming fires were identified correctly, but smoldering fires were problematic.
T. Kaiser, et al., “Temperature fluctuation as a detection criterion,” Fire Safety Journal 29, 217–226, (1997) use a fire detector based on temperature fluctuations to provide an additional criterion for fire detection.
Y. R. Sivathanu, et al., “Fire detection using time series analysis of source temperatures,” Fire Safety Journal 29, 301–315 (1997), have shown that power spectral density and the probability density function of the source temperatures are be sufficient to determine the presence of a fire in the vicinity of the detector.
R. J. Roby, “Multi-signature fire detector,” U.S. Pat. No. 5,691,703 (1997), uses two sensors or detectors to detect two different signatures, and their outputs are compared to predetermined values or combined in a sum or a product which is compared to predetermined reference values. In the case of the sum, the outputs can be multiplied by a weighting coefficient prior to adding the outputs. Smoke and CO are used to evaluate this invention.
J. Y. Wong, “False alarm resistant fire detector with improved performance,” U.S. Pat. No. 5,798,700 (1998), is a continuation of U.S. Pat. No. 5,592,147 (1997). The invention uses a smoke and CO2 sensors to generate a fire alarm when both CO2 and smoke exceed threshold values at the same time, or the rate of increase of CO2 exceeds a predetermined threshold rate.
D. A. Peralta, “Smoke and carbon monoxide detector with clock,” U.S. Pat. No. 5,936,532 (1999), combined a smoke detector with a CO detector for residential use. When the presence of smoke or CO is detected, an alarm is initiated. Capability is provided to manually de-activate the annunciator in case of false alarms. The annunciator is then automatically re-activated after a predetermined time interval.
D. H. Marman, et al., “Fire and smoke detection and control system”, U.S. Pat. No. 5,945,924 (1999), use a fire alarm algorithm based on the rate of change of CO2 and/or smoke. The algorithm has shown reduction in nuisance alarms and response times. The rate of change of CO2 was specified in parts per million per minute (ppm/min).
J. Y. Wong, “Fire detector,” U.S. Pat. No. 5,966,077 (1999), is a continuation of U.S. Pat. No. 5,691,704 (1997) and U.S. Pat. No. 5,767,776 (1998). The invention combines a CO2 detector and a smoke detector to detect the presence of a fire when CO2 rate of increase exceeds a first predetermined level and smoke exceeds a predetermined level, or when the rate of increase of CO2 exceeds a second predetermined rate.
J. Y. Wong, “Method for dynamically adjusting criteria for detecting fire through smoke concentration,” U.S. Pat. No. 6,107,925 (2000), uses CO2 measurements to dynamically adjust the smoke detector output signal fire detection criterion. The CO2 measurements are used to determine the probability of the existence of a fire.
J. Y. Wong, “Fire detector,” U.S. Pat. No. 6,166,647 (2000), combines a smoke detector with an electronic nose that detects fire radicals to detect the presence of a fire. A fire alarm is initiated with the rate of increase of both the smoke and fire radicals exceed predetermined threshold rates.
D. S. Johnston et al., “Carbon monoxide and smoke detection apparatus,” U.S. Pat. No. 6,426,703 (2002), combine a smoke detector and a CO sensor to make a fire alarm algorithm. Smoke and CO outputs are processed independently. When CO exceeds a predetermined limit, without the presence of smoke, alarm sounds.
The present invention improves on the art by using a fire detection system that comprises a smoke detector, a gas sensor for carbon dioxide, a gas sensor for carbon monoxide, and a fire alarm algorithm based on the rates of increase of these three fire signatures. Concentrations of CO and CO2 are usually expressed in parts per million (ppm) and smoke signal in Volt (V). These rates of increase are specified in parts per million per second (ppm/sec) for CO and CO2, and in V/sec for smoke. The decision to alarm is based on the condition when the smoke rate of increase is exceeded, and CO or CO2 rate of increase exceeds its predetermined threshold rate as well. The fire alarm algorithm provides a way to reduce or minimize false alarms generated by smoke detectors alone. Fire detection algorithm is interrogated once per second, offering a fast response to the detection of incipient fires. Furthermore, the algorithm is immune to signal offsets caused by background changes or sensor aging, and noises that are inherent in the measurements of smoke, CO and CO2.